This invention relates to fittings with a ferrule for use in connecting tubes, said ferrule having a forwardly converging conical front part and an inner and outer surface, said inner surface having two annular cutting edges of different diameters. The ferrule or sealing ring is inserted between a connecting piece or body member having an inner conical surface, and a pressure member or nut whereby when the pressure member is tightened, the ferrule is axially moved along the stationary tube end with its cutting edges cutting into the outer tube wall by pushing up wall material of the tube. The ferrule is initially in one piece, but is sheared into two cutting rings during tightening to provide serial ring engagement and a signal to the operator that the fitting assembly is completed.
A tube joint or tube fitting with a ferrule comprising two cutting edges one behind the other in the direction of motion, defining bores with different diameters is disclosed in German Pat. No. 1,296,901. The back face of the rear cutting edge has a conical transition into the cylindrical inner surface of the rearward part of the ferrule deformably joining the tube wall at the last phase of assembly, whereby no exact limiting can be felt by the assembler to indicate that installation is completed. Thereby the danger arises of stripping the thread of the fitting during the operation.
Another known tube joint or tube fitting is disclosed in U.S. Pat. No. 3,120,969 comprising a ferrule having an outer and an inner conical front part, with two cutting edges, the rear cutting edge being a limiting edge with its front side vertically directed to the surface of the pipe or tube. The limiting edge is followed by an inner surface of cylindrical shape changing into a convex curved surface at the end of the ferrule. With this ferrule the limiting edge effects a progressive increase of the cutting resistance when the connection is completed, so that the assembler cannot determine the end of operation clearly enough by the applied torque.
It is well known that the reason for many defects of fittings is not due to the construction but to the installation work or assembly operation. In many cases the assembler uses too much energy thereby stripping the thread. Also, when using thin walled tubes, the tube to be connected may reduce its profile at the point of connection thereby tilting the end of the tube and causing leakages.
Another known tube fitting is disclosed in U.S. Pat. No. 3,893,716 in which a two piece ferrule is sequentially set into a tube surface as the ferrule and tube are moved axially. This form of fitting includes a slotted rear ring having a shear section thereon and requires preassembly of the rings.